Interface TemporalAmount

Framework-level interface defining an amount of time, such as
"6 hours", "8 days" or "2 years and 3 months".

This is the base interface type for amounts of time.
An amount is distinct from a date or time-of-day in that it is not tied
to any specific point on the time-line.

The amount can be thought of as a Map of TemporalUnit to
long, exposed via getUnits() and get(TemporalUnit).
A simple case might have a single unit-value pair, such as "6 hours".
A more complex case may have multiple unit-value pairs, such as
"7 years, 3 months and 5 days".

There are two common implementations.
Period is a date-based implementation, storing years, months and days.
Duration is a time-based implementation, storing seconds and nanoseconds,
but providing some access using other duration based units such as minutes,
hours and fixed 24-hour days.

This interface is a framework-level interface that should not be widely
used in application code. Instead, applications should create and pass
around instances of concrete types, such as Period and Duration.

Implementation Requirements:

This interface places no restrictions on the mutability of implementations,
however immutability is strongly recommended.

getUnits

Returns the list of units uniquely defining the value of this TemporalAmount.
The list of TemporalUnits is defined by the implementation class.
The list is a snapshot of the units at the time getUnits
is called and is not mutable.
The units are ordered from longest duration to the shortest duration
of the unit.

Implementation Requirements:

The list of units completely and uniquely represents the
state of the object without omissions, overlaps or duplication.
The units are in order from longest duration to shortest.

addTo

Adds the amount to the specified temporal object using the logic
encapsulated in the implementing class.

There are two equivalent ways of using this method.
The first is to invoke this method directly.
The second is to use Temporal.plus(TemporalAmount):

// These two lines are equivalent, but the second approach is recommended
dateTime = amount.addTo(dateTime);
dateTime = dateTime.plus(adder);

It is recommended to use the second approach, plus(TemporalAmount),
as it is a lot clearer to read in code.

Implementation Requirements:

The implementation must take the input object and add to it.
The implementation defines the logic of the addition and is responsible for
documenting that logic. It may use any method on Temporal to
query the temporal object and perform the addition.
The returned object must have the same observable type as the input object

The input object must not be altered.
Instead, an adjusted copy of the original must be returned.
This provides equivalent, safe behavior for immutable and mutable temporal objects.

The input temporal object may be in a calendar system other than ISO.
Implementations may choose to document compatibility with other calendar systems,
or reject non-ISO temporal objects by querying the chronology.

This method may be called from multiple threads in parallel.
It must be thread-safe when invoked.

Parameters:

temporal - the temporal object to add the amount to, not null

Returns:

an object of the same observable type with the addition made, not null

// these two lines are equivalent, but the second approach is recommended
dateTime = amount.subtractFrom(dateTime);
dateTime = dateTime.minus(amount);

It is recommended to use the second approach, minus(TemporalAmount),
as it is a lot clearer to read in code.

Implementation Requirements:

The implementation must take the input object and subtract from it.
The implementation defines the logic of the subtraction and is responsible for
documenting that logic. It may use any method on Temporal to
query the temporal object and perform the subtraction.
The returned object must have the same observable type as the input object

The input object must not be altered.
Instead, an adjusted copy of the original must be returned.
This provides equivalent, safe behavior for immutable and mutable temporal objects.

The input temporal object may be in a calendar system other than ISO.
Implementations may choose to document compatibility with other calendar systems,
or reject non-ISO temporal objects by querying the chronology.

This method may be called from multiple threads in parallel.
It must be thread-safe when invoked.

Parameters:

temporal - the temporal object to subtract the amount from, not null

Returns:

an object of the same observable type with the subtraction made, not null